Thermally Stable Polypropylene Superhydrophobic Surface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

A Thermally Stable Polypropylene Superhydrophobic Surface Due to the Formation of a Surface Crystalline Layer of Microsized Particles Cuiyun Zhang, Wei Chen, Junren Chen, Xiaoling Wu, and Xinping Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06842 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

A Thermally Stable Polypropylene Superhydrophobic Surface Due to the Formation of a Surface Crystalline Layer of Microsized Particles Cuiyun Zhang, Wei Chen, Junren Chen, Xiaoling Wu, Xinping Wang*

Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China. E-mail: [email protected]

ABSTRACT: Polypropylene is a potential material for the fabrication of superhydrophobic surfaces. In this paper, two isotactic polypropylenes made from a metallocene catalyst (miPP) and Ziegler-Natta catalyst (ziPP) were employed to prepare superhydrophobic surfaces. The results showed that the superhydrophobicity and thermal stability of the miPP film were much better than those of the ziPP film. The water contact angle (CA) and sliding angle (SA) of the miPP film were 155° and 10°, respectively, after being heated at 100 ºC for 5 h, while the water CA and SA of the ziPP film were 135° and 90°, respectively. This difference in the thermal stability was attributed to the miPP easily forming a surface crystalline layer on the microsized particles, which resulted in an increased roughness and improved thermal stability. These results were supported

by

the

dyeing

experiment, high-resolution

transmission

electron

microscopy (HRTEM) images and selected-area electron diffraction patterns. Furthermore, the morphology and roughness of the spin-coated miPP and ziPP films were explored by atomic force microscopy (AFM), and the results also showed that the good thermal stability of miPP film was resulted from the increased crystallinity.

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1. INTRODUCTION In the past twenty years, superhydrophobic surfaces have drawn great scientific and industrial interest due to their applications in water repellent, self-cleaning, and antifouling materials.1-3 Conventionally, the hydrophobicity of a surface can be enhanced in two ways:4-6 one is to create a rough surface, and the other is to modify the surface with materials that have a low surface free energy, such as fluorinated compounds. Most superhydrophobic surfaces have been obtained by controlling the surface topography with various processing methods,7 such as gel-like roughened polypropylene8 and polyethylene prepared with solvent processes,9, 10 densely packed aligned carbon nanotubes,11-13 aligned polyacrylonitrile nanofibers,14 phase separation of polymer blends,15 and lithography16. In general, there are two criteria to evaluate superhydrophobic surfaces: a very high water contact angle (CA > 150º) and a very low sliding angle (SA < 10º) 4 , which can also be expressed in terms of the difference between advancing and receding contact angle, i.e., hysteresis. To date, most studies have focused on improving the CA and SA, while the investigation on stability of superhydrophobic surfaces in harsh environments, which is a very important factor for various materials in practical application was little concerned. The thermal stability is the main challenge in the development of superhydrophobic materials. Han et al.9 reported that the microporous surface of low-density polyethylene disappeared in 20 min upon heating, and the wettability changed from superhydrophobic to hydrophobic. Polymers with a high glass transition temperature (Tg) have the potential to be superhydrophobic materials with high thermal stability. Previous articles reported that fluorinated polyimide could withstand extreme temperatures up to 300 °C and that polybenzoxazine could remain superhydrophobic at temperatures higher than 150 °C.17,

18

Unfortunately, materials

with high Tg have shortcomings, such as high cost and poor processability. Additionally, the superhydrophobicity of an organic-inorganic composite can be maintained at 90 °C,19 but its weak adhesion to the substrate and poor wearability are obstacles to its application. Thus, it is urgent to find an economical, thermal stable and high-performance superhydrophobic film. 2


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Polypropylene (PP) is the most popular thermoplastic polymer and is experiencing rapid expansion in production with an average annual growth rate of 9% since 1991. Superhydrophobic coatings made by PP have great potential applications in many fields, such as in the vehicle industry (e.g., as tapes and composites) and geotextiles. Recently, the development of metallocene, which is a single-site catalyst, has been fueled in part the product diversification of polyolefins.20,

21

These

organometallic compounds can control the chain architecture, such as by the manipulation of stereo-defects in isotactic polypropylene (iPP). The degree of crystallinity of the polymers can also be influenced by metallocenes.20 Moreover, metallocene polymerization chemistry results in narrow molecular weight distributions and eliminates certain spurious side reactions that plague conventional heterogeneous Ziegler-Natta reactions.22, 23 For example, Rosa et al. investigated the crystallization behavior of isotactic polypropylene from a metallocene catalyst and found that the miPP crystallized in a continuum of disordered modifications intermediate between the α and γ forms, and the amount of disorder in the crystals depended on the stereoregularity of the sample and the crystallization conditions.21 Subsequently, they further indicated that the difference in the polymorphic behavior of the metallocene and Ziegler-Natta iPP samples was related to the distribution of defects in the polymeric chains generated by the different kinds of catalytic systems.20 Although many studies20-26 discussed iPPs generated by metallocene and Ziegler-Natta catalysts, most of them focused on the crystallization behavior of the miPP and ziPP. It is well known that the surface properties of superhydrophobic coatings are closely related to their crystallization behavior. Erbil et al.8 described a simple and inexpensive method for forming a superhydrophobic coating using PP and a suitable selection of solvents and temperature to control the surface roughness. These

results

suggest

that

iPPs

are

potential

materials

for

generating

superhydrophobic coatings. However, the surface properties of iPPs, especially those obtained by metallocene polymerization chemistry, have not received a substantial amount of attention. Whether the surface properties of iPP films produced by Ziegler-Natta catalysts are the same as those generated by metallocene catalysts are 3



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still not understood. In this paper, the superhydrophobic surfaces of iPP films produced by Ziegler-Natta and metallocene catalysts were investigated. The results showed that the superhydrophobic surface made from the miPP had a much lower SA and higher thermal stability than that made from ziPP. Then, the surface morphology at different temperatures was observed by scanning electron microscopy (SEM), which indicated that the different superhydrophobic and thermally stable behaviors arose from the micro-nano structure of the film. Furthermore, the crystallinity of the film was studied by the dyeing and HRTEM. The results showed that the surface crystallinity of the miPP was higher than that of the ziPP. To investigate the detailed surface crystallization behaviors of the surface, the spin-coated films of miPP and ziPP were studied by AFM. Compared with that of the ziPP, the surface of miPP was covered by small crystals. This study substantially improved the understanding of the formation mechanism of thermally stable miPP superhydrophobic surfaces.

2. EXPERIMENTAL SECTION 2.1 Materials The miPP (Achieve PP3825, Mn = 39000 g/mol and Mw/Mn = 2.0) and ziPP (Escorene PP4062, Mn = 57000 g/mol and Mw/Mn = 2.4) used in this work were produced by the Exxon Chemical Company. The physical properties were described in detail elsewhere.24, 26 The granular miPP and ziPP materials were used without any further treatment. Xylene and methyl ethyl ketone (MEK) (Shanghai Chemical Company, China) were chosen as the solvents. 2.2 Sample Preparation The iPP film was prepared according to the reported method by Erbil.8 The miPP and ziPP were dissolved slowly in a xylene/MEK (6:4 v/v) mixture at an initial concentration of 20 mg/mL at 100 ºC. A few drops of the polymer solution were then placed onto the glass slides. The samples were dried at 25 ºC for 24 h and then annealed at 30 ºC in a vacuum oven for one week to remove the remnant solvents. 2.3 Characterization 4


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The water CA was measured with a sessile water drop using a Krüss DSA10-MK2 contact angle measuring system at ambient temperature. The SA was measured by using the reported method.9 First, a water drop was placed gently on a level surface (tilt stage), and then the surface was slowly tilted. When the droplet rolled off the surface, the angle of the sample stage was the SA. For the measurement of the thermal stability of the films, the samples were first heated at 100 ºC for different times in air and then cooled to room temperature before the CA measurements. The microstructures of the iPP films were observed using a JSM-5610LF (JEOL Co., Japan) scanning electron microscope. The samples were coated with a 20-30 Å layer of Au prior to observation. The dyeing experiment was carried out on the iPP films in aqueous media containing 0.1 wt% disperse orange S-4RL at pH 5 adjusted by acetic acid at 100 ºC for various dyeing times. The uptake of the dye was obtained by measuring the absorbance of the diluted dye solution at the maximum absorption wavelength of the dye with an Cary 300 Bio ultraviolet-visible spectrophotometer (Varian Inc., CA, USA). The uptake percentage (U) was calculated with the following equation: U= (A0-At) × 100%/A0 where A0 and At represent the absorbance of the dye solution before and after the dyeing process for a certain time, respectively. Morphology observations and analysis of the crystallinity of the film surface were performed on a JEOL JEM-2010 HRTEM operated at 200 kV that was equipped with a liquid anticontamination trap and a Gatan image enhancer. A high-resolution pole piece (SHP) with a spherical aberration coefficient (Cs) of 0.5 mm was used. A minimum-dose system (MDS) with a small beam size (spot size 3) and low beam intensity was employed to minimize radiation damage to the specimen during examination. The images obtained from HRTEM were recorded on LeKai photographic plates with a direct magnification of 500,000. The observed region of the sample constituted the edge of the microparticles, which guaranteed that the image reflected the morphology of the film surface. 5



The surface morphology of the spin-coated iPP films was observed by a Multimode-8 AFM (Bruker Co., USA) using tapping mode. The iPP was dissolved in xylene at a concentration of 20 mg/mL. The film was prepared on a SiO2 substrate by spin-coating at 9000 rpm. The films were kept in a vacuum oven at 30 ºC to remove the residual solvent. Then, the samples were heated at 180 ºC for 20 sec and quenched in liquid nitrogen. The quenched films were annealed at 140 ºC for 70 min to observe the thermal stability.

3. RESULTS AND DISCUSSION 3.1 Superhydrophobic Surfaces of miPP with Excellent Thermal Stability 165

100 90

155

80 70

145

60 135

50 40

125

30 20

115

Sliding angle (degree)

Contact angle (degree)

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10 105

0 0

1

2

3 4 Time (h)

5

6

Figure 1. Water CAs (filled) and SAs (hollow) of ziPP (circle) and miPP (triangle) films as a function of thermal treatment time. The xylene/MEK (3:2 v/v) mixture was used as the solvent. Temperature: 100 ºC.

On the basis of Erbil’s work,8 ziPP films were fabricated by solution casting. The water CA of the film was 158.8º, as shown in Figure 1, which is similar to the reported value of approximately 160°.27-30 However, this film was not truly superhydrophobic due to a high SA (32°). The thermal stability of the ziPP surfaces was further investigated. As shown in Figure 1, the water CA of the ziPP film decreased to 136.7º when it was heated at 100 °C for 5 h, and the SA noticeably increased to approximately 90º. A SA close to 90º means that the water droplet on the surface cannot slide off even though the surface is tilted until it is vertical; that is, the 6


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water droplet is “pinned” to the surface. The water CA of the miPP was 163.5º, and the SA was approximately 4º (Figure 1). The water drops could roll off the surface when it was slightly tilted, similar to water drops on lotus leaves. The high water CA and the low hysteresis confirmed the true superhydrophobic surface of the miPP film. As shown in Figure 1, the water CA of miPP was still above 155º, and the SA was 9º when the film was annealed at 100 ºC for 5 h, indicating the excellent thermal durability of the superhydrophobic surface. This is a very interesting phenomenon, which suggests that some important physical properties of PP can be improved by metallocene chemistry. (b)

(a) 156.5°±2.3°

163.5°±1.5°

A

A

5 μm

(d)

(c) 136.7 °±2°

158.8 ° ± 1.8°

N-iPP

(e)

Figure 2. SEM micrographs of (a) miPP and (c) ziPP films from a solution of 20 mg/mL in xylene/MEK (3:2 v/v). SEM micrographs of (b) miPP and (d) ziPP films heated at 100 ºC for 5 h. (e) SEM micrograph at high magnification of area marked with A in (a).

The surface morphology of the iPP films was investigated by SEM. The result in Figure 2(a) illustrates that the miPP film surface was entirely composed of microsized particles with a length of approximately 1 µm. Figure 2(e) shows a magnified image of the area marked with an A in Figure 2(a), showing that the microsized particles 7



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look like nanoflowers and are similar in appearance to the papillae on a natural lotus leaf. However, the surface microstructure of the ziPP film (Figure 2(c)) was quite different from that of the miPP film. It appeared that the porous surface of the ziPP was composed of compressed fibers, which resulted in a smoother surface than that of the miPP. The self-cleaning property of lotus leaves is caused by a cooperative effect of the micro- and nanoscale structures on their surface,6,

25, 31, 32

which means that the

superhydrophobicity of a surface is closely related to the structure of the film on it. The surface roughness is determined by the ratio of the actual rough surface area to the geometrically projected surface area, which can be effectively enhanced by the micro- and nanostructures. The relationship between the CA and the surface roughness can be described by the equation established by Cassie and Baxter as follows:4, 33 cos θr = f1 cos θ –f2 Here, θr and θ are the CAs on the iPP surface with a rough and flat structure, respectively, and f1 and f2 are the fractions of the interface areas of the iPP surface in contact with a liquid and air, respectively (f1+f2 = 1). It is easy to deduce from this equation that increasing the value of f2, i.e., increasing the fraction of air on the surface, leads to an increase in θr. Accordingly, the air trapped in the rough surface can significantly decrease the contact area between the water and solid surface of the miPP, which results in a high water CA and extremely low SA. The SEM images of the iPP films heated at 100 °C for 5 h are shown in Figure 2(b) and (d). The changes in the morphology of the miPP (Figure 2(b)) were not very obvious after annealing. However, as shown in Figure 2(d), a substantial change in the surface morphology of the ziPP film occurred after annealing. The compressed fibers were melted, and the microstructure of the ziPP film became more compact after heating, which resulted in a sharp decrease in the roughness and a corresponding decreased amount of trapped air. The decreasing water CA of the ziPP films heated at 100 °C resulted from a decrease in the roughness.

8


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3.2 The Mechanism about Thermal Durability of miPP Superhydrophobic Surface It has been reported that crystallization behavior plays an important role in the pore formation, microstructure, and wetting properties of iPP and LDPE films.9, 10 The crystallization is also considered to be the main reason for high-strength welds in metallocene PP/polyethylene laminates.26 With the aim of revealing the surface microstructure of both miPP and ziPP films, HRTEM was employed to study the crystallinity. Figure 3 shows the HRTEM images of miPP and ziPP films together with selected-area electron diffraction patterns. A typical one-dimensional lattice image and clear electron diffraction pattern obtained from miPP film are presented in

(a)

(b)

Figure 3. HRTEM images of iPP films: (a) miPP and (b) ziPP. The inserts show the selected-area electron diffraction patterns.

Figure 3(a), in which a lattice spacing of 0.32 nm is resolved and corresponded to the d(200) of crystalline α-phase PP, which is consistent with the results reported by Loos and Zhou.34, 35 However, as shown in Figure 3(b), no obvious one-dimensional lattice image was observed for the ziPP film, which was also confirmed by electron diffraction patterns. The results indicated that the miPP and ziPP films presented different surface structures. The miPP film surface exhibited a good crystal structure, 9



while the structure of the ziPP film surface was amorphous. 8

Dye uptake (%)

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6

4

2

0 0.0

0.7

1.4

2.1

2.8

3.5

Time (h) Figure 4. Dye uptake on miPP (●) and ziPP (■) films as a function of the dyeing time. The inserts are pictures of dyed films at the selected dyeing time.

Since surface crystallinity limits the diffusion of dye molecules to the inside of the film, the dyeability of the film covered with crystals is weaker than that of the amorphous film.36-40 The dyeing experiment was employed to illustrate the difference in the ziPP and miPP films, and the results are presented in Figure 4. The dye uptake of the miPP and ziPP films increased steadily with time, but the uptake capacity of miPP film was poorer than that of the ziPP film. It was very obvious that the dyed ziPP film was much darker than that of the miPP film when the dyeing time was the same, per the inserts in Figure 4, indicating that the dye molecules diffused more easily into the ziPP film than into the miPP film. In other words, the surface crystallinity of the miPP film was higher than that of the ziPP film. The special interfacial properties of miPP were also reported by Bates and coworkers.26 They revealed that the metallocene-catalyzed polyolefins had higher joint strengths than the Ziegler-Natta-based PE/iPP laminates, which was attributed to the accumulation of amorphous polymers at the ziPE/ziPP interface.

10


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(a)

(b)

20 10 -20 0

5

10

15

20 0

5

10

15

20

(d)

(c)

20 10 -20 0

5

10

15

20

0

5

10

15

20

Figure 5. AFM images of (a) miPP and (c) ziPP films prepared by spin-coating and quenching in liquid nitrogen. The quenched (b) miPP and (d) ziPP heated at 140 ºC for 70 min. Typical height profiles across the sample surface are given beneath each figure.

The microstructure of a film surface influences its thermal durability.41, 42 It was reported that the roughness of a film is directly related to the crystallinity of the polymer during film formation from solution.43-45 To obtain the surface structure at nanoscale and determine the relationship between the roughness and thermal durability, AFM was employed to measure the morphology and roughness of the spin-coated iPP films. The surface topography and surface height profiles of the spin-coated miPP and ziPP films are shown in Figure 5. It was obvious that the quenched miPP film was composed of homogeneous and separate crystalline particles, 11



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while the ziPP film was covered by nonuniform particles (Figure 5(a) and (c), respectively). The surface height profiles in Figure 5 show that the roughness of the miPP was higher than that of the ziPP. This result indicated that the crystallization rate of the miPP was much higher than that of ziPP, which resulted in the higher crystallinity of the miPP. As shown in Figure 5(b) and (d), after heating at 140 ºC for 70 min, the morphology of the miPP remained nearly constant, while the crystals on the ziPP increased in size. Because the crystallinity of the spin-coated miPP was higher than that of the ziPP, its thermal durability was better. The remarkable variability in the thermal durability and surface properties of the miPP and ziPP can be attributed to the polymerization catalysts. Ziggler-Natta polymers contain substantially higher amounts of noncrystallizable material than metallocene polymers due to the exceptional control of metallocene catalysts.26

Figure 6. Illustration of the change in the surface topography of the miPP and ziPP films heated at 100 ºC.

The results from the dyeing experiment, HRTEM micrographs and AFM images reveal that the degree of crystallization of the miPP surface was higher than that of the ziPP surface, which resulted in a higher thermal durability of the miPP superhydrophobic surfaces. An explanation for this is illustrated in Figure 6. Since the miPP had a high crystallization rate, the miPP film readily formed a layer of crystalline particles on the surface during film formation (solution-to-solid process). 12


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However, the ziPP film surface constituted an amorphous phase due to the relatively slow crystallization rate. Since there was an amorphous phase on the ziPP surface, the surface layer melted, and the molecular chains easily interpenetrated into neighboring microparticles during heating. The micro-protuberance formed by the ziPP particles disappeared during heat treatment at 100 ºC (shown in Figure 2(d)). Multilevel micro-hole structures collapsed accordingly, and the roughness decreased, which resulted in a remarkable decrease in the CA and an increase in the SA. However, due to the crystalline structure formed on the microparticle surface of the miPP film, upon heating, these condensed miPP chains did not interpenetrate with other microparticles, and the microparticle surface layer did not melt at 100 ºC. Therefore, the microsized particles appeared to have structure like nanoflowers on the miPP film surface and remained constant; correspondingly, their superhydrophobic properties were maintained.

4. CONCLUSIONS In conclusion, a superhydrophobic iPP surface with good thermal durability was prepared using miPP. The surface of the miPP was covered with microsized particles that differed from that of the ziPP. The dyeing experiment and the HRTEM micrographs showed that microsized crystalline particles formed on the surface of the miPP film, while the ziPP particle surface exhibited an amorphous state. Furthermore, the morphology and roughness of the spin-coated iPP film obtained by AFM indicated that the miPP had a higher crystallization rate and higher crystallinity during film formation (solution-to-solid transition process) than the ziPP, resulting in higher roughness. The good thermal durability of the miPP superhydrophobic surface was attributed to the crystalline structure formed by the microparticle surface of the miPP film. The condensed miPP chains did not interpenetrate with other microparticles, and the surface layer of the microparticle surface did not melt and join together at 100 ºC. Therefore, the microsized particles on the miPP film surface appeared to have a structure like that of nanoflowers and remained constant; correspondingly, their superhydrophobic properties were maintained. Our findings provide a new field of 13



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application for miPP and a new way to improve the thermal durability of superhydrophobic surfaces.

AUTHOR INFORMATION Corresponding Author *Tel/fax:

86–571–8684–3600.

Email:

[email protected]

or

[email protected] ORCID Xinping Wang: 0000-0002-9269-3275

NOTES The authors declare no competing financial interest

ACKNOWLEDGMENTS The authors acknowledge financial support from the Natural Science Foundation of Zhejiang Province (No. LQ19B040002), the National Nature Science Foundation of China (No. 21873085) and the Science Foundation of Zhejiang SciTech University (No. 17062155-Y).

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Nanocarbon-Induced

Rapid

Transformation

of

Polymer

Surfaces

into

Superhydrophobic Surfaces. ACS Appl. Mater. Interfaces 2010, 2, 3378-3383.

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Thermally Stable Polypropylene Superhydrophobic Surface

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